Airborne particle detection with selective thermophoretic particle deflection

ABSTRACT

A method for analyzing particles in an air stream includes concentrating the particles in an interior region of the air stream and deflecting the concentrated particles in the air stream with a generated thermal gradient. Smaller particles in the air stream may be selectively deflected away from the interior region and towards a periphery of the air stream at a different rate than larger particles in the air stream. The generated thermal gradient may be controlled to deflect particles in a selected particle size range onto a surface of a particle detector. An effective mass of the collected particles and an aerosol mass concentration estimate of the particles within the selected particle size range may be generated. Systems for analyzing particles are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplications. 62/586,130; 62/586,134; 62/586,141; 62/586,143; and62/586,148; all filed on Nov. 14, 2017; the entire contents of each arehereby incorporated herein by reference.

TECHNICAL FIELD

This patent disclosure relates generally to the field ofparticulate-matter detection and more specifically to air-qualitysensors and to systems and methods for determining airborne-particlecontent.

BACKGROUND

The presence of airborne and other gas-borne particulate matter (PM),alternatively referred to as aerosol particles, can contribute to poorair quality and potentially adverse health effects. These particles canpenetrate into human and animal lungs, contributing to lung disease,heart disease, cancer, and other illnesses. Such particles may beproduced by many sources, including industrial and agriculturalprocesses, fossil-fuel combustion in power plants and vehicles, fires,smoking, and other natural and manmade causes.

Airborne particles with a diameter of 2.5 microns or less (often termedPM2.5) tend to be particularly problematic. These finer sizedparticulates can remain suspended in the air for long periods of timeand can penetrate deep into the lung alveoli. Airborne particles under0.1 microns in diameter can pass through the lungs and enter the body,causing damage to other organs. Particles of intermediate sizes, such asbetween 2.5 and 10 microns (often termed PM10), although not aspotentially toxic as the smaller PM2.5 particles, are also medicallyproblematic because these can also penetrate into at least the outerportions of the lungs. In contrast, the larger sized particles, such asparticles over 10 microns in diameter, tend to be less problematic froma health perspective. This is because such larger particles do notpenetrate as deeply into the lungs and tend to settle out of the airrelatively quickly. The impact of nanoparticles in the range of 0.01microns to 0.1 microns is relatively unknown and is an active area ofstudy, although significant adverse health impacts are suspected.

Monitoring and controlling airborne particulate matter is of intenseinterest due to potentially adverse health and environmental effects.Various health, legal, government, scientific, industrial and commercialentities have considerable interest in methods of monitoring airborneand other gas-borne particulate matter. Methods that can furtherdistinguish between various sizes of particulate material areparticularly valued. Current systems for monitoring particulate mattertend to be relatively bulky, complex and expensive, which generallyrender them unsuitable for mass-market use.

Airborne particles can range greatly in size, shape, content andconcentration. Methods and systems for selectively fractionating andseparating airborne particles before particle detection and propertydeterminations can provide benefits including reductions in the size,complexity and cost of such systems, improved device performance anddetection rates, and lower limits of detection.

BRIEF SUMMARY OF THE INVENTION

In some embodiments, the invention may be a method for analyzingparticles in an air stream includes concentrating particles in aninterior region of the air stream and deflecting the concentratedparticles in the air stream with a generated thermal gradient. Smallerparticles in the air stream may be selectively deflected away from theinterior region and towards a periphery of the air stream at a differentrate than larger particles in the air stream.

The generated thermal gradient may be controlled to deflect particles ina selected particle size range onto a surface of a particle detector.The airstream velocity of the air stream may be controlled. Deflectedparticles within a selected particle size range may be collected on asurface of a particle detector. The selected particle size range mayinclude a particle size range between about 0.01 microns and 0.1microns, 0.01 microns and 0.3 microns, 0.1 microns and 1.0 microns, 1.0microns and 2.5 microns, 2.5 microns and 10.0 microns, and 10.0 micronsand larger, or other particle size ranges of interest. The particledetector may include one or more of a bulk acoustic wave (BAW)resonator, a thin-film bulk acoustic wave resonator (FBAR), a solidlymounted resonator (SMR), a quartz crystal microbalance (QCM), awall-mounted particle detector, a time-of-flight detector, a resonantsensor, a capacitive sensor, an infrared sensor, an optical sensor, a UVsensor, and a particle mass detector. An effective mass of the particlescollected on the surface of the particle detector may be determined. Anaerosol mass concentration estimate of the particles within the selectedparticle size range may be generated and provided to a requestingentity.

In some embodiments, the invention may be a system for analyzingparticles that includes an inlet, a particle concentrator fluidicallycoupled to the inlet, and a particle discriminator fluidically coupledto the particle concentrator. The particle discriminator includes an airchannel for containing an air stream. The air channel may extend fromthe inlet through the particle concentrator and through the particlediscriminator.

A controller may be electrically coupled to the particle concentratorand to the particle discriminator and configured to allow concentratingparticles in an interior region of the air stream and deflecting theconcentrated particles in the air stream with a generated thermalgradient. Smaller particles in the air stream may be selectivelydeflected away from the interior region and towards a periphery of theair stream at a different rate than larger particles in the air stream.The controller may be further configured to allow controlling thegenerated thermal gradient to deflect particles in a selected particlesize range onto a surface of a particle detector, controlling anairstream velocity of the air stream in the air channel, collectingdeflected particles within a selected particle size range on a surfaceof a particle detector, determining an effective mass of the particlescollected on the surface of the particle detector, generating an aerosolmass concentration estimate of the particles within the selectedparticle size range, and providing the aerosol mass concentrationestimate.

In some embodiments, the invention may be a non-transitorycomputer-readable medium storing computer-readable program code to beexecuted by at least one processor for analyzing particles in an airstream includes instructions configured to cause concentrating particlesin an interior region of the air stream and deflecting the concentratedparticles in the air stream with a generated thermal gradient. Smallerparticles in the air stream may be selectively deflected away from theinterior region and towards a periphery of the air stream at a differentrate than larger particles in the air stream. The instructions mayfurther cause controlling the generated thermal gradient to deflectparticles in a selected particle size range onto a surface of a particledetector, controlling an airstream velocity of the air stream in the airchannel, collecting particles within a selected particle size range on asurface of the particle detector, determining an effective mass of theparticles collected on the surface of the particle detector, generatingan aerosol mass concentration estimate of the particles within theselected particle size range, and providing the aerosol massconcentration estimate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of a system for analyzingparticles in an air stream.

FIG. 2 illustrates a method of analyzing particles traversing a systemfor analyzing particles in an air stream.

FIG. 3 illustrates the elements and operation of a thermophoreticparticle concentrator.

FIG. 4 illustrates the elements and operation of a thermophoreticparticle discriminator.

FIG. 5 illustrates a method of fractionating and discriminatingconcentrated particles in an air stream.

FIG. 6 shows a plot of particle position across an air channel versusdistance along the air channel having a thermophoretic particleconcentrator and a thermophoretic particle discriminator.

FIG. 7 shows a block diagram of a system for analyzing particles in anair stream.

FIG. 8 depicts an exploded view of a system for analyzing particles inan air stream.

FIGS. 9A-91 illustrate top and cross-sectional views of variousthin-film heater elements for use in systems for analyzing particles.

FIGS. 10A-10C illustrate perspective and cross-sectional views of aresonant-based particle detector.

FIGS. 11A-11B illustrate the operation of a system for analyzingparticles in an air stream.

FIGS. 12A-12B illustrate a top view and a side view of a system foranalyzing particles in an air stream.

FIGS. 13A-13B illustrate a perspective view and a side view of a systemfor analyzing particles.

FIGS. 14A-14D illustrate top and side views of a system with arectangular air channel for analyzing particles in an air stream andoperation thereof.

FIGS. 15A-15D illustrate top and side views of a system with anexpanding air channel for analyzing particles in an air stream andoperation thereof.

FIG. 16 illustrates a top view of a system for analyzing particles in anair stream including a centrifugal particle separator stage.

FIGS. 17A-17B illustrate top and side views of a system for analyzingparticles with a widening air channel and a narrowing channel height.

FIGS. 18A-18B illustrate top and side views of a thermophoretic particledetection system for analyzing particles including a pair ofmulti-tapped heater elements extending through the particle concentratorand the particle discriminator.

FIG. 19 illustrates a block diagram of a system for analyzing particlesin an air stream.

FIG. 20 shows a block diagram of a method for analyzing particles in anair stream.

FIG. 21 shows a block diagram of a method for analyzing particlesincluding the generation of an aerosol mass concentration estimate.

DETAILED DESCRIPTION OF THE INVENTION

The techniques, methods, devices, and systems disclosed herein mayresult in smaller, simpler and lower cost airborne particle detectionand monitoring devices that allow mass-market use in homes, buildings,workplace environments, industrial facilities, indoor and outdoorenvironments, and personal air-quality monitors. Improved airborneparticle detection systems and methods may be used in a variety ofdevices including cellular phones, smartphones, laptop and tabletcomputers, thermostats, voice-activated tabletop monitors, wearabledevices such as watches and personal health monitors, air monitors forgreen buildings and home-automation systems, vehicle cabin monitoring,smoke detectors and protective devices such as face masks andeyeglasses, among other applications.

Improved air particle monitoring devices can be facilitated by systemsand methods configured to use thermophoretic forces. Such systems andmethods can produce various effects useful for such improved devices byemploying suitable thermal gradients. These effects may includeconcentrating and focusing particulate material, fractionatingparticulate material according to size, and directing various sizes ofparticulate material onto one or more suitable particle sensors such asresonant-based MEMS sensors or other devices in a controlled manner.

Thermophoretic force generally refers to the force that may be exertedon small particles such as micron and sub-micron sized particles thatare suspended in a gas or fluid media in the presence of thermalgradients. Absent thermal gradients (also referred to as “temperaturegradients” or “heat gradients”), suspended particles experience normalrandom Brownian motion. In the presence of thermal gradients, moreenergetic molecules of the gas or fluid media may impact one side of theparticle relative to the other side of the particle, producing a netforce on the particle that varies as a function of the particlediameter, temperature gradient, gas pressure, particle temperature andother variables such as the thermal conductivity and heat capacity ofthe particle. This thermophoretic force can, in turn, impart athermophoretic velocity to such particles that varies as a function ofthe thermal gradient, gas viscosity, gas density and the size andcomposition of the particles. The thermophoretic force may be used toconcentrate particles in an interior region of an air stream and toselectively deflect the particles towards suitable particle detectorsfor detection and analysis.

Airborne particle detection may be accomplished with a thermophoreticparticle discriminator in combination with a particle concentrator.Particles in an interior region of an air stream may be deflected bythermophoretic forces generated from thermal gradients in the airstream. Smaller particles in the air stream may be selectively deflectedaway from the interior region of the air stream and towards a peripheryof the air stream at a higher rate than larger particles, allowingdifferentiation of the airborne particles when collected by one or moreparticle detectors. Controlling the thermal gradients in the sample airstream and the airstream velocity in an air channel allows particles ina particular size range to be selectively deflected by thethermophoretic particle discriminator. Particles in a selected particlesize range may be deflected, collected, detected and analyzed by one ormore particle detectors such as resonant-based particle detectors.

While the embodiments disclosed herein generally refer to systems andmethods for analyzing particles in an air stream, “air”, althoughencompassing normal earth atmospheric air, can be any type of gas orfluid traversing the air channel. Particles in the air stream generallyrefer to micron or sub-micron sized particles with a plurality ofparticle sizes and composition that are suspended in the air stream andare generally distinct from the smaller gas molecules or atomscomprising the carrier gas. A micron (also referred to as “μm”) is aunit of length equal to one micrometer or one-millionth of a meter.

Similarly numbered elements in the various figures below apply tosimilar elements. While the figures are intended to be illustrative,dimensions and features of the various elements shown in the figures arenot always drawn to scale for clarity.

FIG. 1 shows a simplified block diagram of a system 100 for analyzingparticles in an air stream. System 100 includes an inlet 110, a particleconcentrator 120 and a particle discriminator 130. A set of data andcontrol signals 140 may be used for communicating, sending and receivingdata, and controlling system 100. Inlet air stream 112 may includeparticulate matter that may be concentrated by particle concentrator 120and then deflected, collected, detected and analyzed by particlediscriminator 130 and associated analytical components. Outlet airstream 118 may include air drawn from the inlet 110 absent particlescollected by particle discriminator 130.

In some implementations, system 100 may be wired or wirelessly coupledto a cell phone, smartphone, laptop, or other computerized devices, toenable one or more processors within the system 100 or computerizeddevice to process the data from system 100 and produce temporal andspatial (if multiple such systems are used) measurements of particulatematter content. In some implementations, system 100 may becommunicatively coupled to one or more processors in a data center or acloud computing environment to perform data processing of theparticulate matter data and to provide results of particulate matterdata procured from system 100.

FIG. 2 illustrates a method of analyzing particles 114 traversing asystem 200 for analyzing particles in an air stream. Particles 114 inthe inlet air stream 112 passing through the inlet 110 are drawn throughan air channel 102 (also referred to as a “flow channel”) that isencompassed by one or more channel walls 104. Without any focusing orconcentration, particles 114 generally follow an air stream 106 (alsoreferred to as a “flow stream”) in the air channel 102 along one or morestreamlines 108. Particles deflected by applied forces such asthermophoretic force or gravitational force may be deflected away from aparticular streamline 108 in the direction of the applied force.

Airborne particles 114 may have different sizes and shapes and comprisedifferent materials or aggregates of various materials. Various types ofairborne particles 114 at the inlet air stream 112 are depicted andlabeled 1 through 8, indicating larger spherical particles 1 and 5,smaller spherical particles 2 and 6, fibrous particles 3 and 7, andfractured particles 4 and 8. As these particles 114 traverse the airchannel 102, thermophoretic forces from heater elements 122 a, 122 b(also referred to as “heating elements” or “focusing heaters”) in theparticle concentrator 120 may force particles 114 away from a peripheryof the air channel 102 near the channel walls 104 and towards aninterior region 220 of the air channel 102 that may be at or near thegeometrical center of the air channel 102. Thermophoretic particleconcentrator heater elements 122 a, 122 b may be used to focus incomingparticles 114 into a tighter beam to facilitate analysis. Particles 114concentrated in an interior region 220 of air channel 102 may progresstowards the particle discriminator 130 along one or more streamlines108.

Particle discriminator 130 may include one or more heater elements 132(also referred to as a “precipitation heater” or a “deflection heater”)to deflect and collect particles on a wall 104 or particle detector 134opposite the heater element 132. Particles 114 may be deflected bythermal gradients generated by the thermophoretic particle discriminatorheater element 132 and directed towards a particle detector 134 near thewall 104 where particles in a selected particle size range may becollected on a surface 136 of the particle detector 134. The collectedparticles 116 may cause a shift in a resonant frequency of the particledetector 134, which may be calibrated to determine the amount of masscollected on the surface 136. Fractionation, collection, detection andanalysis of particles 114 in a particular size range depend in part onsize-dependent and composition-dependent thermophoretic forces and aninitial well-collimated and pre-focused stream of incoming particles114. Uncollected particles may continue down the air channel 102 alongone or more streamlines 108 and exit via the outlet air stream 118.

FIG. 3 illustrates some of the elements and operation of athermophoretic particle concentrator 120. Thermophoretic particleconcentrator 120 may include at least one pair of thermophoreticparticle concentrator heater elements 122 a, 122 b positioned near aperiphery of an air channel 102 within the particle concentrator 120.Particle concentrator 120 may include an air channel 102 with a firstopen end for inlet air stream 112 and a second open end for outlet airstream 118 that allows particles 114 and air stream 106 to traverse theair channel 102. The air channel 102 may be enclosed by one or morechannel walls 104 extending from at least the first open end to thesecond open end. Two or more heater elements 122 a, 122 b may bepositioned between the first open end and the second open end. Theheater elements 122 a, 122 b may be positioned near a periphery of theair channel 102 and cooperatively configured to thermophoretically forceparticles 114 in the air stream 106 away from the periphery and towardsan interior region 320 of the air channel 102.

In some embodiments, a position-dependent heat profile may be generatedalong the length of the air channel 102 to drive particles 114 towardsthe center of the air channel 102, while otherwise allowing the airstream 106 to remain unperturbed. Particles 114 in the air stream 106may be thermophoretically forced towards the interior region 320 of theair channel when the heater elements 122 a, 122 b are heated and thermalgradients emanating from heater elements 122 a, 122 b are generated.Isothermal lines 324 a, 324 b show lines of constant temperature withinthe air stream 106. As power applied to heater elements 122 a, 122 bchanges and as the velocity of air stream 106 is adjusted, the positionand the shape of isothermal lines 324 a, 324 b may change.Thermophoretic forces 326 a, 326 b may be generated and act on particles114 as particles 114 in the air stream 106 traverse the air channel 102.Thermophoretic forces 326 a, 326 b increase as the thermal gradients inthe air channel 102 increase and act on particles 114 in the directionof most rapid thermal decrease, that is, in a direction perpendicular tothe isothermal lines 324 a, 324 b and with a magnitude proportional tothe gradient of the temperature in the air stream 106. When acted uponby thermophoretic forces 326 a, 326 b generated by the thermal gradientsin the air stream 106, particles 114 may be forced away from theperiphery and towards an interior region 320 of the air channel 102. Theparticles 114 may be deflected across streamlines 108 towards the centerof the air channel 102, increasing the concentration of particles 114 inthe interior region 320 of the air channel 102. In some implementations,the temperatures of the heater elements 122 a, 122 b may be varied tocontrol the position of the concentrated particles 114 within theinterior region 320 as the particles 114 continue downstream in the airchannel 102.

While the interior region 320 of the particle concentrator 120 (and ofthe particle discriminator 130) is generally centered on the geometriccenter of the air channel 102 and includes portions of the air channel102 in the vicinity of the geometric center, in some implementations theinterior region 320 may be centered about an axis that is offset fromthe centerline of the air channel 102. Dimensions of the interior regionare generally on the order of the width and/or length of the activesensor area of the particle detector 134, which may be 200 microns orless. As particle fractionation and collection improves with a tighterdistribution of particles in the interior region of the air stream 106,device performance improves with a tighter particle distribution. Insome implementations, the interior region may be defined as the spatialvolume where a simple majority of airborne particles 114 in the airstream 106 are concentrated. In some implementations, the interiorregion may be defined as a percentage of the channel volume, such as 1%,2%, 5%, 25% or 50% of the channel volume. In some implementations, theinterior region may include the geometric center of the air channel anda percentage of the area about the geometric center, such as 1%, 2%, 5%,25% or 50% of the cross-sectional area at a point along the centerlineof the air channel.

FIG. 4 illustrates the elements and operation of a thermophoreticparticle discriminator 130. The particle discriminator 130, sometimesreferred to as a deposition chamber or a settling chamber, includes anair channel 102 having one or more channel walls 104 that contain an airstream 106. The air channel 102 in the particle discriminator 130 mayextend from the inlet 110 through a particle concentrator 120 andthrough the particle discriminator 130. The particle discriminator 130may be fluidically coupled to the particle concentrator 120 and to theinlet 110. Particles 114 in the air channel 102 may be concentrated inan interior region of the air stream 106 within the particleconcentrator 120 and then passed via the fluidically coupled air channel102 into the particle discriminator 130.

The particle discriminator 130 may include at least one thermophoreticparticle discriminator heater element 132. Heater element 132 may beconfigured to deflect particles 114 in an interior region of the airstream 106 towards a peripheral wall 104 of an air channel 102 thatencompasses the air stream 106. A thermal gradient in the air channel102 generated by the heater element 132 may thermophoretically forceparticles 114 towards the peripheral wall 104 in a directionperpendicular to the air stream 106. Isothermal lines 434 a, 434 b showlines of constant temperature. The position and shape of isothermallines 434 a, 434 b may change as the power applied to heater element 132changes and as the velocity of air stream 106 is adjusted.

As particles 114 enter particle discriminator 130, the thermophoreticforce generated by one or more heater elements 132 in the particlediscriminator 130 may force particles 114 towards a periphery of the airstream 106 near one of the channel walls 104 of the particlediscriminator 130. Thermophoretic forces 436 a, 436 b may be generatedand act on particles 114 as particles 114 in the air stream 106 traversethe air channel 102. Thermophoretic forces 436 a, 436 b increase as thethermal gradient in the air channel 102 increases and act on particles114 in the direction of greatest decrease in temperature with respect toposition. When acted upon by thermophoretic forces 436 a, 436 bgenerated by the thermal gradients in the air stream 106, particles 114are forced away from the interior region of the air channel 102 andtowards a periphery wall 104.

The particles 114 may be deflected away from the interior region of theair channel 102 and towards a particle detector 134 positioned on theperiphery wall 104 of the air channel 102. The particle detector 134 maybe configured to collect particles 114 deflected from the interiorregion of the air stream 106. Deflected particles 114 may be collectedon a surface 136 of a particle detector 134 where the collectedparticles 116 may be detected and analyzed. Smaller particles in the airstream 106 may be selectively deflected away from the interior regionand towards a periphery of the air stream 106 at a different rate thanlarger particles in the air stream. Particles may be subject toarea-dependent viscous and drag forces and to gravitational forces thatare density and volume dependent. Heavier particles, for example, may beselectively deflected by gravitational forces at a higher rate thanlighter particles of similar size.

Power applied to the heater element 132 of the particle discriminator130 may be controlled to generate and control thermal gradients in theair channel 102 to deflect particles 114 in a selected particle sizerange onto a surface 136 of the particle detector 134. The selectedparticle size range may include a particle size range between about 0.01microns and 0.1 microns, 0.01 microns and 0.3 microns, 0.1 microns and1.0 microns, 1.0 microns and 2.5 microns, 2.5 microns and 10.0 microns,and 10.0 microns and larger, or other particle size ranges of interest.Particles 114 not collected by the particle discriminator 130 maycontinue downstream along one or more streamlines 108 and through theoutlet air stream 118. In some implementations, power applied to heaterelement 132 may be dithered, modulated or otherwise varied to moreevenly spread the distribution of collected particles 116 on the surface136 of the particle detector 134. In some implementations, the powerapplied to heater element 132 may be varied sequentially in time tocollect and analyze particles of a first size during a first timeperiod, then collect and analyze particles of a second size during asecond time period, and so forth.

FIG. 5 illustrates a method of fractionating and discriminatingconcentrated particles 114 in an air stream 106. A cross-sectional viewof a particle discriminator 130 is shown with an air channel 102 andchannel walls 104 encompassing the air channel 102 and the air stream106. Particles 114 from an inlet air stream 112 that are concentrated inan interior region of the air stream 106 may be deflected towards aperipheral wall 104 of the air channel 102 with one or more heaterelements 132 in the particle discriminator 130. The particles may bedeflected with a thermal gradient generated by the heater element 132,with smaller particles 552, 554 in the air stream 106 deflected awayfrom the interior region and towards a periphery of the air stream 106at a higher rate than larger particles 556, 558 in the air stream. Theamount of transverse displacement of the various particles may change inan amount that varies roughly inversely with the size of the airborneparticles. That is, smaller sized particles are generally deflected morethan larger sized particles. The trajectory of smaller particles underthe influence of thermophoretic forces is more readily altered comparedto larger particles under the same influences, allowing localizedparticle detectors to distinguish between different particle sizes. Therate of deflection may also be a function of mass, thermal conductivity,surface area, and other properties of the deflected particles. Forexample, denser particles of similar size may be selectively deflectedat a different rate than less dense particles.

A plurality of particle detectors 134 a, 134 b, 134 c may be positionedon a channel wall 104 of the particle discriminator 130 to allowcollection and detection of particles within one or more particle sizeranges. The lighter (smaller) particles may be collected by the firstparticle detector 134 a and the heavier (larger) particles may becollected by the second or third particle detectors 134 b, 134 c,allowing the particle discriminator 130 to distinguish between differentparticle sizes. The physical mechanism for such size fractionation anddiscrimination assumes some form of pre-focusing or concentration of theparticulate matter stream within the air channel 102 to prevent a givenparticle detector 134 a, 134 b, 134 c from collecting a mixture ofdifferent particle sizes from different heights or locations in thesample air stream 106. In the absence of active measures to concentratethe particles 114 into a beam, the particles 114 may be distributedthroughout the entire vertical profile of air channel 102. Thus largeparticles 114 positioned near the bottom of the air channel 102 may bedeflected onto the first particle detector 134 a while smaller particles114 near the top of the air channel 102 may be deflected onto the secondparticle detector 134 b, interfering with the ability of the particlediscriminator 130 to distinguish between different particle sizes orother particle properties.

Power to the heater element 132 may be controlled to allow collection ofthe deflected particles in a particular particle size range onto asurface of one of the particle detectors 134 a, 134 b, 134 c. Asillustrated in FIG. 5, smaller particles 552′ may be collected onparticle detector 134 a; somewhat larger particles 554′ may be collectedon particle detector 134 b; larger particles 556′ may be collected onparticle detector 134 c; and the largest particles 558′ may remainuncollected and continue along the air stream 106 through the outlet airstream 118, effectively generating a cutoff point for particlecollection.

FIG. 6 shows a plot 600 of particle position across an air channelversus distance along the air channel having a thermophoretic particleconcentrator 120 and a thermophoretic particle discriminator 130.Particles in the air stream initially distributed uniformly across thechannel height enter the particle concentrator 120 and are forcedtowards the interior region of the air stream by heater elements 122 a,122 b as previously described. The concentrated particles traverse theair channel towards a particle discriminator 130. The particlediscriminator 130 is positioned in a downstream direction of the airstream with respect to the particle concentrator 120.

Under normal atmospheric pressure, smaller particles, such as particlesunder about 2.5 microns in size, are minimally affected by gravity.These particles tend to move through the air channel with minimaldeflection due to gravity. Absent any other forces, these particlesfollow almost the same path as the surrounding air in the air channel.To detect and measure suspended aerosol particles, a precipitatingthermophoretic force may be applied to the particles to deflect theparticle's trajectory enough so that the particle may be propelledtowards and collected on the surface of one or more particle detectors134 a, 134 b, 134 c, 134 d.

The particle discriminator 130 may have at least one heater element 132and an array of particle detectors 134 a, 134 b, 134 c, 134 d disposedon a wall of the air channel opposite the heater element 132 that isconfigured to collect deflected and fractionated particles. Lighter andsmaller particles 632 may be deflected at a higher rate and collected byparticle detector 134 a. Heavier and larger particles 634 may bedeflected at a lower rate and collected by particle detector 134 d.Still heavier and larger particles 636 may remain uncollected and may bepassed along the air channel where they may be deflected downwards bygravitational forces acting on the particles.

Particles smaller than a particular cutoff size, such as 2.5 microns indiameter, may be selectively collected and analyzed by the particlediscriminator 130 and particles larger than the cutoff size may continuepast the particle discriminator 130. Heavier and larger particles 636above the cutoff point that are near the periphery of the air channelmay experience a lower local velocity of air than that of particles nearthe center of the air channel, allowing gravitational forces more timeto act on the heavier particles. The effect of gravity and theorientation of the particle discriminator 130 can affect the cutoffsize.

Because the thermophoretic force can be made much greater than the forceof gravity, at least for particles sized less than about 10 microns, theparticle sensors may be configured on nearly any surface of the airchannel including (relative to the force of gravity) the floor, theceiling, or the sides of the air channel. Although in this disclosurefloor and ceiling mounted particle detectors are mainly discussed, thisis not intended to be limiting. Given the comparatively large impactthat precipitation heaters can have on particle trajectories, particlesensors can also be mounted on the sides of an air channel (relative tothe force of gravity) or upside down as desired.

In some implementations, a thermophoretic particle detection system maybe configured with one or more additional stages of particleconcentrators 120 and particle discriminators 130 positioned downstreamof a first stage. Thermophoretic forces act on smaller particles anddeflect the smaller particles at a faster rate than larger particles inthe air stream. The first stage allows concentration of particles in theair stream and collection of particles in a first particle size rangewhile passing particles outside of the first range. A second stageallows re-concentrating and re-focusing of particles remaining in theair stream and subsequent collection of particles in a second range witha larger particle size range than the first stage. Additional stageswith pairs of particle concentrators 120 and particle discriminators 130may be added. Each stage may include one or more focusing heaters in theparticle concentrator 120 and one or more deflection heaters andparticle detectors in the particle discriminator 130 to redirect largerparticles towards the center of the air channel that may bere-concentrated and re-deflected for additional collection and analysis.In some implementations, multiple stages of particle concentrators anddiscriminators allow further separation of resonant-based particledetectors resulting in improved acoustic isolation and less mechanicalcoupling between detectors for improved sensitivity.

FIG. 7 shows a block diagram of a system 700 for analyzing particles inan air stream. System 700 includes an inlet 110, a particle concentrator120 fluidically coupled to the inlet 110, and a particle discriminator130 fluidically coupled to the particle concentrator 120. The inlet 110,particle concentrator 120 and particle discriminator 130 include an airchannel 102 extending through the inlet 110, particle concentrator 120and particle discriminator 130 for containing an air stream 106. The airchannel 102 includes one or more channel walls 104 for containing theair stream 106. One or more heater elements 122 a, 122 b may bepositioned on opposing sides of the air channel 102 within thethermophoretic particle concentrator 120. One or more heater elements132 may be positioned on a channel wall 104 of air channel 102 withinthe particle discriminator 130. One or more particle detectors 134 maybe positioned on a wall 104 of the particle discriminator 130 oppositethe heater elements 132 to collect and detect particles. The particledetectors 134 may include one or more piezoelectric layers 734 withelectrode layers 732, 736 on each side of the piezoelectric layer 734.

System 700 may include one or more controllers 150. Controller 150 mayinclude one or more processors configured to allow concentratingparticles 114 in an interior region of the air stream 106 within theparticle concentrator 120 and deflecting the concentrated particles 114in the air stream 106 within the particle discriminator 130 withgenerated and controlled thermal gradients in the air channel 102.Controller 150 may include various electronic circuits, passive devices,metal traces, electrical interconnects and wires for sending signals toand receiving signals from heater elements 122 a, 122 b, heater element132 and particle detector 134. Electrical power and ground connectionsfor controller 150 may also be provided.

Controller 150 may include circuitry to operate the particle detector134 in a resonant mode and to detect changes in the resonant frequency.The circuitry may include signal amplifiers and preamplifiers, signalconditioning circuitry such as filters, mixers, local oscillators,demodulators, phase-lock loops, counters, A-D (analog to digital)convertors, and divide-by-n circuits, and control circuitry to determinethe frequency shifts associated with mass loading from collectedparticles on the surface of the particle detector 134. The controller150 may include processing circuitry to process data from the particledetector 134 and to analyze properties of the collected particles.

Smaller particles 114 in the air stream 106 may be selectively deflectedaway from the interior region of the air channel 102 and towards aperiphery of the air stream 106 at a higher rate than larger particles114 in the air stream 106. The controller 150 may be configured to allowcontrolling the generated thermal gradient to deflect the particles in aselected particle size range onto a surface 136 of the particle detector134. The controller 150 may be configured to draw air and to control anairstream velocity of the air stream 106 in the air channel 102. Thecontroller 150 may be configured to allow collecting particles 114within a selected particle size range on the surface 136 of particledetector 134. The controller 150 may be configured to determine aneffective mass of the particles 114 collected on the surface 136 of theparticle detector 134. The controller 150 may be configured to generatean aerosol mass concentration estimate of the particles 114 within theselected particle size range and provide or send the generated aerosolmass concentration estimate via the data and control signals 140 toanother location. The controller may be configured to correct orcompensate for temperature, relative humidity, ambient pressure, andother factors. The controller may be configured to operate in thesemanners using non-transitory computer-readable medium storingcomputer-readable program code to be executed by at least one processorassociated with the controller for analyzing particles in an air streamthrough the use of associated program code including associated programinstructions.

FIG. 8 depicts an exploded view of a system 800 for analyzing particlesin an air stream 106. System 800 includes an inlet 110 having an inletport 812 with an open end 814 for an inlet air stream 112. System 800may include an outlet port 818 with an open end 816 for an outlet airstream 118. An air channel 102 may encompass the air stream 106 andextend from the open end 814 to the open end 816. The size and shape ofthe open ends 814 and 816 may vary depending on the device andapplication. For example, circular open ends 814 and 816 may have adiameter between about 30 microns and about 50 millimeters. Rectangularopen ends 814 and 816 may have a width between about 30 microns andabout 50 millimeters and a height between about 30 microns and about 50millimeters.

The thermophoretic particle concentrator 120 may have two heaterelements 122 a, 122 b positioned on opposite sides of the air channel102. A thermophoretic particle discriminator 130 may have one or moreheater elements 132 positioned on one side of the air channel 102 and aparticle detector 134 positioned on the opposite side of the air channel102. In the implementation shown in FIG. 8, the air channel 102 iscircular and the heater elements 122 a, 122 b and 132 are positionedalong a circumference of the channel wall in a direction substantiallyperpendicular to the air stream 106 flowing in the air channel 102.Particles in the air stream 106 may be thermophoretically forced towardsan interior region 820 of the air channel 102 when the heater elements122 a, 122 b are heated and thermal gradients emanating from the heaterelements 122 a, 122 b are generated. Heater elements 122 a, 122 b andheater element 132 may include one or more thin-film heater elements,resistive films, resistive segments, heater wires, or other heatertypes. For economic and packaging reasons, the same heater type may beused in either or both the particle concentrator 120 and the particlediscriminator 130, although each heater will generally operate at adifferent temperature depending on their use as a focusing heater or aprecipitation heater. The operating temperature may vary depending inpart on the shape and placement of the heater elements, the resistivityof the heater elements, and the applied power. Operating temperaturesfor heater elements in the thermophoretic particle concentrator 120 aretypically between about 20 degrees centigrade and 50 degrees centigradeabove ambient temperature. Operating temperatures for heater elements inthe thermophoretic particle discriminator 130 are generally higher andare typically between about 50 degrees centigrade and 200 degreescentigrade above ambient temperature for effective control of particlemovement. The temperature of the heater elements 122 a, 122 b and 132and the thermal gradients generated therefrom may be controlled bycontrolling the electrical power applied to each of the heater elements,such as by controlling the amount of electrical current passed throughthe heater elements or by controlling the voltage applied across theterminals of the heater elements.

One or more banded heater elements 822 a, 822 b may be positioned on oraround portions of the inlet port 812. The banded heater elements 822 a,822 b allow circular or rectangular inlet ports 812 to be surroundedwith heater elements that extend around the entire inlet wall 104. Thebanded heater elements 822 a, 822 b may be configured with heatersegments disposed on opposite sides of the air channel 102 thatencompasses the air stream 106. The Power applied to inlet heaterelements 822 a, 822 b may generate thermophoretic forces acting onparticles in the incident air stream 106, forcing the particles awayfrom the walls 104 of the air channel 102 towards an interior region 820of the air stream 106 and beginning the particle concentration process.Further concentration of particles in the air stream 106 may occur inthe thermophoretic particle concentrator 120 downstream of the inlet airstream 112. Temperatures generated by the banded heater elements 822 a,822 b may be as low as a few degrees above ambient temperature to deterparticles from collecting on surfaces of the inlet port 812. The bandedheater elements 822 a, 822 b may be configured to control or limit thelevel of moisture or humidity in the air channel 102 during use.

Data and control signals 140 may be communicated to and from system 800with one or more electrical connectors and signal lines coupled tosystem 800. The air channel 102 includes one or more channel walls 104.The sides of the air channel 102 will normally be surrounded by walls104, except at the open ends for the inlet air stream 112 and the outletair stream 118. At least one particle detector 134 may be positionedalong the wall 104 and configured so that the airborne particles, afterbeing propelled transversely by the precipitation heat gradient producedby the precipitation heater element 132, may be detected by the particledetector 134.

While thermophoretic particle discriminators 130 are generally used asexemplary embodiments in the various descriptions contained herein,other detection devices and methods such as particle sensors thatoperate by laser light scattering are also possible and are notdisclaimed by this disclosure. The embodiments described herein mayoperate with many different types of particle sensors. MEMS-basedparticle sensors and in particular an FBAR sensor or an addressablearray of FBAR sensors are presented as a particular type of particledetector that may be used.

Air or other carrier gas may be drawn through the air channel 102 withan air movement device (not shown) generally positioned downstream ofthe outlet air stream 118 to generate the air stream 106. The airmovement device may include a pump, blower, fan, turbine, motorized airintake device, bellows pump, membrane pump, peristaltic pump, pistonpump, positive-displacement pump, rotary vane pump, Venturi device,airflow management device, or other air drawing means for moving ordrawing air through the air channel 102. In some implementations, theair velocity of the inlet air stream 112 in the system 800 may beadequate without requiring active airflow generation by using a passiveairflow mechanism such as an ambient air flow, an ambient pressure dropacross the length of the air channel, or a convective heat process.

Particulate filters (not shown) and protective screens may be usedupstream of the particle concentrator 120 to filter out particularlylarge particles and to protect the air channel 102 from accumulatingundesirable debris. In some implementations, a size-selective particlefilter or a size-selective input device may be included as part of theinlet 110 or placed upstream of the inlet 110. In some implementations,the walls 104 of the air channel 102 may be mechanically supported andmounted in a way that provides for improved thermal isolation. In someimplementations, the air channel 102 may be suspended in an aerogel thatprovides good mechanical robustness and good thermal isolation.

FIGS. 9A-91 illustrate top and cross-sectional views of variousthin-film heater elements for use in systems for analyzing particles.Thin-film heater element 900 shown in FIG. 9A includes a thin heaterlayer 910 such as a patterned layer of nickel, chrome-nickel orplatinum. Electrical bond pads 940 and 950 allow electrical connectionsto the heater layer 910 via heater contact regions 944 and 954,respectively. One or more electrical traces 946 may be used tofacilitate electrical connections between bond pad 940 and contactregion 944. Similarly, one or more electrical traces 956 may be used tofacilitate electrical connections between bond pad 950 and contactregion 954. The illustrative cross-sectional view shown in FIG. 9B takenthrough dashed lines AA′ in FIG. 9A shows heater layer 910 positionedbetween two insulating layers 912 and 916 such as layers of silicondioxide or silicon nitride. The heater layer 910 may be positioned abovea cavity region 918 defined in a heater substrate 920 to reduce powerloss through the heater substrate 920. Bond wires 942 and 952, or otherconnective wiring, may be used to make connections to bond pads 940 and950, respectively.

FIG. 9C shows a thin-film heater element 900 with heater layer 910patterned in a serpentine manner having various heater segments 922a-922 f serially connected to each other and electrically connected toelectrical bond pads 940 and 950. Alternative implementations ofthin-film heater elements include metal foil formed into electricallyconnected serpentine heater segments on polyimide film backing such asKapton tape with patterned electrodes, or other microfabricated heaterelements such as polysilicon heater elements formed on fused quartzsubstrates. In some implementations, other sources of heat such ashigh-resistivity wire, heat pipes, radiant heating or inductive heatingmay be used to generate the thermal gradients.

FIG. 9D illustrates a cross-sectional view of a thermally isolatedwall-mounted thin-film heater element 900 with a polymeric barrier layer904 that may serve as a channel wall of an air channel. The wall-mountedthin-film heater element 900 in FIG. 9D presents no structural featuresin the air channel except for the relatively smooth outer surface of thepolymeric barrier layer 904, minimizing the level of any airflowdisruptions in the air stream. A thin heater layer 910 may be formedseparately on a plastic heater substrate 920 and laminated or otherwiseattached to the barrier layer 904 with an adhesive layer 906 such as aUV-curable adhesive or epoxy. The heater layer 910 may be electricallyconnected to bond pads 940 and 950 with one or more electrical traces946 and 956, respectively. The bond pads 940 and 950 may be attachedwith anisotropic conductive film (ACF) 980 to electrical interconnectsformed on one or more interconnect layers 962, 964 through one or moreplated flex via holes 966 and dielectric layers 968 included in aflexible printed circuit board 960 (also referred to as a “flex”). Theconstruction shown with a cutout region in one of the flex layers 970generates a cavity region 918 between the heater layer 910 and theunderlying flex layer 972 that allows a higher level of thermalisolation and lower operating power to achieve the same operatingtemperature. Additional cavity regions (not shown) may be formed inunderlying flex layers 972, 974 of the flexible printed circuit board960 to achieve additional thermal isolation. Thermal isolation of thethin-film heater layer 910 may result in improved temperature control,less temperature variation, and lower operating power. The cavity region918 may be filled with an aerogel or other thermally insulating materialto provide mechanical strength in addition to thermal isolation.

FIG. 9E illustrates a top view of a multi-tapped thin-film heaterelement 900. The multi-tapped thin-film heater element 900 in FIG. 9Emay be attached to a polymeric barrier layer that in turn may serve as achannel wall of an air channel. The heater layer 910 may be disposed ona plastic heater substrate 920 and electrically connected to bond pads940, 950 via the contact regions 944, 954 and electrical traces 946,956, among others. Each heater segment 922 g through 922 r between twoadjacent heater taps may be individually controlled by the voltagesapplied across each segment to allow control of a temperature profile inan adjacent air stream 106. Voltages between adjacent heater taps canstep up or step down in voltage level as desired to control the powerapplied to the heater segment between the adjacent heater taps. Settingthe electrical potential difference to zero across any two adjacentheater taps reduces the thermal generation between the two adjacentheater taps to zero, allowing temperature zone control andflow-dependent temperature distributions along the length of themulti-tapped heater element. Multi-tapped heater elements require fewerelectrical connections compared to individually tapped heater elementsand allow closer-spaced and continuous heater segments for improvedtemperature profile control. One or more pairs of multi-tapped heaterelements may be formed on the heater substrate 920. The heater segmentsbetween any two heater taps may be formed in any one of a variety ofshapes including straight segments, curved segments, angled segments,tapered segments, serpentine segments and segments with varying widths.One or more stub heater segments 9221, 922 r may be included on thesubstrate 920 with independent electrical access to allow additionalcontrol over the temperature profile and thermal gradients generated inthe air stream 106.

FIG. 9F through FIG. 9I show top views of various thin-film heaterelements 900 with thermally coupled heat spreaders. Heat spreaders arethermally conductive structures that heat up when nearby thermallycoupled heater elements or heater segments are heated up. The heatspreaders may heat up to temperatures that are generally less than thetemperature of the associated heater element, allowing improved controlof the temperature distribution across the air channel 102 as an airstream 106 passes by the heater elements and heat spreaders. The heatspreaders may or may not carry current and are largely passive devices.While the heat spreaders may be mechanically and electrically connectedto and in some implementations be formed from the same material as theheater elements, the heat spreaders may be fully passive devices thatare electrically isolated from the heater elements yet close enough toextract thermal energy from the heater elements and redistribute thethermal energy throughout other portions of the air channel. Thequantity and shape of the heat spreaders may vary from heater to heateror from segment to segment within the same air channel. For example, apair of triangular heat spreaders 938 a, 938 b may be thermally coupledto heater segments 922 s, 922 t positioned near a channel wall 104 of anair channel 102 to selectively heat up air or other gas in the airstream 106 flowing through the air channel 102, as shown in FIG. 9F. Anarray of spike-shaped heat spreaders 938 c, 938 d, 938 e, 938 f amongothers may be thermally coupled to heater segments 922 u, 922 v, asshown in FIG. 9G. A thermally coupled heat spreader 938 h that extendsacross the air channel 102 to heater segments 922 w, 922 x may betapered or otherwise contoured between the heater segments 922 w, 922 x,as shown in FIG. 9H. Heat spreader 938 i may extend between and overlapassociated heater segments 922 y, 922 z, as shown in FIG. 9I. Heatertaps 928 a through 928 p may provide electrical connectivity to each ofthe heater elements or heater segments shown in FIG. 9F through FIG. 9I.One or more passive metallic heat shunts (not shown) may be configuredon one or more layers of a multi-layer flexible printed circuit board toserve as a thermal load and to alter the dynamic temperature responseresulting in higher and more controlled thermal gradients in the airchannel. One or more heat sinks (not shown) may be included to maintaina desired temperature such as an ambient temperature along one or moreportions of the air channel.

Various heat spreaders, stub heaters, heat shunts, and heat sinks may becombined with one or more multi-tapped thin-film heater elements andcontrol electronics to generate the desired thermal gradients in the airchannel for focusing, concentrating, deflecting and collecting particlesin the air stream. Thermal potential wells generated in the air streamwith control of the thermal fields from the various heat spreaders, stubheaters, heat shunts, heat sinks, heater segments, and heater elementscan effectively garner and capture particles in the air stream 106 fordetection and analysis.

FIGS. 10A-10C illustrate perspective and cross-sectional views of aresonant-based particle detector 134. The resonant-based particledetector 134 may be an acoustic resonator device such as quartz crystalmicrobalance or a film bulk acoustic resonator having a surface exposedto air or gas-borne particles. Particles collected on the surface of theresonator may change the resonant frequency of the resonant device. Thechange in the resonant frequency due to the additional mass loading maybe detected electronically. The resonant device may operate in afrequency range between a few megahertz and several gigahertz, with adetectable frequency shift on the order of a few Hertz that generallyshifts downwards as mass is added.

The particle detector 134 may include one or more of a bulk acousticwave (BAW) resonator, a thin-film bulk acoustic wave resonator (FBAR), asolidly mounted resonator (SMR), a quartz crystal microbalance (QCM), awall-mounted particle detector, a time-of-flight detector, a resonantsensor, a capacitive sensor, an infrared sensor, an optical sensor, a UVsensor, or a particle mass detector. In some implementations, theparticle detector may include a one-dimensional or two-dimensional arrayof such sensors, or more than one type of particle sensor. The particledetector may be positioned on or near a wall of an air channel 102 andmay be arranged under or near a heater element 132 of a particlediscriminator 130. In some implementations, particles in a selectedparticle size range may be deflected and collected on a surface 136 ofthe particle detector 134. The particle detector 134 may be used todetermine an effective mass and other properties of the particlescollected on the surface such as an aerosol mass concentration estimate.

In the implementation of FIG. 10A, a depiction of an FBAR-based particledetector 134 is shown including bond pads 1010 a, 1010 b, 1010 cdisposed on a substrate 1020 with an FBAR 1030 suspended over a cavity1038 and a portion of the collected particles 116 on a surface 136 ofthe FBAR 1030. Bond pads 1010 a, 1010 b, 1010 c may be used to makeelectrical connections such as signal and ground to the FBAR 1030. Thecross-sectional view of the FBAR 1030 in FIG. 10B shows the collectedparticles 116 on the surface 136 of the FBAR 1030. The FBAR 1030 mayinclude a piezoelectric layer stack having a piezoelectric layer 1034,an upper electrode 1036, and a lower electrode 1032 suspended over acavity 1038 with the FBAR 1030 suspended partially over a cavity region1038 formed in the substrate 1020. The cavity 1038 may be formedunderneath the piezoelectric layer stack to improve the acousticisolation and reduce energy loss to the substrate 1020. One or moredielectric layers 1022, 1024 such as a layer of silicon dioxide orsilicon nitride may be used to provide electrical isolation for the bondpads 1010 a, 1010 b, 1010 c and various electrical traces 1046, 1056positioned between the FBAR electrodes 1032, 1036 and the bond pads 1010a, 1010 b, 1010 c. In some implementations, the electrodes 1032, 1036and electrical traces 1046, 1056 may comprise one or more layers ofaluminum or molybdenum.

FIG. 10C illustrates a cross-sectional view of an acoustically isolatedwall-mounted particle detector 134 with a polymeric barrier layer 1004that may serve as one of the channel walls of an air channel. Thewall-mounted particle detector 134 presents no structural features inthe air channel except for the relatively smooth outer surface of thepolymeric barrier layer 1004, minimizing the level of any airflowdisruptions in the air stream. The particle detector 134 may include anFBAR 1030 having a piezoelectric layer stack with a piezoelectric layer1034, an upper electrode 1036, and a lower electrode 1032 suspended overa cavity region 1038 in the substrate 1020. One or more dielectriclayers 1022 may be used to provide electrical isolation for the bondpads 1010 a, 1010 b and various electrical traces 1046, 1056 positionedbetween the FBAR electrodes 1032, 1036 and the bond pads 1010 a, 1010 b.The FBAR 1030 may be laminated or otherwise attached to the barrierlayer 1004 with an adhesive layer 1006 such as a UV-curable adhesive orepoxy. The bond pads 1010 a and 1010 b may be attached with anisotropicconductive film (ACF) 1080 to electrical interconnects formed on one ormore interconnect layers 1062, 1064 through one or more plated flex viaholes 1066 and dielectric layers 1068 included in a flexible printedcircuit board 1060. The construction is shown with a cutout region intwo of the flex layers 1070 and 1072 generates a cavity region 1018between the substrate 1020 and the underlying flex layer 1074 thatallows a higher level of mechanical and acoustic isolation for theparticle detector 134. Mechanical isolation of the particle detector 134may result in improved sensitivity to added mass and less acoustic andmechanical coupling to other components.

Thermophoretic particle detection systems may include one or moreflex-based wall-mounted heater elements such as that shown in FIG. 9Dand one or more flex-based wall-mounted particle detectors 134 such asthat shown in FIG. 10C. Flex-based air channels may be formed bycombining the flex-based heater elements and the flex-based particledetectors with suitable flex-based sidewalls to form a rectangular airchannel with continuous, smooth walls and surfaces through the inlet,particle concentrator and particle discriminator. For example, one ormore layers of polyimide may be combined with the multi-layer flexassemblies and be used as the polymeric barrier layer 904 and 1004 andas the side walls of the air channel for a compact, low-profile airborneparticle detector.

FIGS. 11A-11B illustrate the operation of a system 1100 for analyzingparticles in an air stream 106. FIG. 11A shows a cross-sectional view ofa system 1100 having an inlet 110, a thermophoretic particleconcentrator 120 and a thermophoretic particle discriminator 130. Aninlet air stream 112 entering an air channel 102 between walls 104 atvarious local velocities 1112 forms a local velocity profile 1114 thatcan vary across the width, height and length of the air channel 102 yetgenerally has a higher local velocity near the center of the air channel102 that diminishes to nearly zero near the walls 104 of the air channel102.

When heater elements 122 a, 122 b on opposite sides of the particleconcentrator 120 are heated, thermal gradients are generated throughoutthe air channel 102, which in turn generate thermophoretic forces 1126a, 1126 b that are perpendicular to isothermal lines 1124 a, 1124 b andpoint generally in the direction of the steepest negative thermalgradient. Particles in the air stream 106 may be directed away from aperiphery of the air channel 102 in the particle concentrator 120 andtowards an interior region of the air channel 102.

When heater elements 132 on one side of the particle discriminator 130are heated, thermal gradients are generated throughout the air channel102, which in turn generate thermophoretic forces 1136 a, 1136 b thatare perpendicular to isothermal lines 1134 a, 1134 b and point in thedirection of the steepest negative thermal gradient. Particles in theair stream 106 within the air channel 102 may be directed away from aninterior region of the air channel 102 in the particle discriminator 130towards a periphery of the air channel 102.

As shown in FIG. 11B, particles 1152, 1154, 1156 and 1158 withincreasing particle size are thermophoretically forced towards aninterior region of the air stream 106 in the particle concentrator 120and then are deflected away from the interior region of the air stream106 in the particle discriminator 130 towards a periphery of the airstream 106, with smaller particles undergoing greater deflection thanlarger particles. In FIG. 11B, smallest particle 1152′ is deflected andstrikes a wall 104 of the particle discriminator 130 before the particledetector 134; small particle 1154′ strikes and is collected on a surfaceof the particle detector 134; large particle 1156′ is not collected bythe particle detector 134 and continues in the air stream 106; andlargest particle 1158′ continues in the air stream 106 with lessdeflection than smaller particles.

FIGS. 12A-12B illustrate a top view and a side view of a system 1200 foranalyzing particles in an air stream 106. System 1200 includes an inlet110, a thermophoretic particle concentrator 120 and a thermophoreticparticle discriminator 130. The thermophoretic particle concentratorincludes an air channel 102 between channel walls 104 having a firstopen end for an inlet air stream 112 and a second open end for an outletair stream 118. The air channel 102 is enclosed by channel wall 104extending from at least the first open end to the second open end. Twoor more heater elements 122 a, 122 b may be positioned between the firstopen end and the second open end and are positioned near a periphery ofthe air channel 102. A cross-section of the air channel 102 and channelwall 104 perpendicular to the air stream 106 is rectangular, and atleast two heater elements 122 a, 122 b are positioned on two opposingsides of the channel wall 104. The heater segments 1222 a of heaterelements 122 a, 122 b extend along the channel wall 104 in a directionsubstantially parallel to the air stream 106. The particle discriminator130 including one or more heater elements 132 and particle detectors 134is coupled to the air channel 102 in a direction downstream from theparticle concentrator 120.

FIGS. 13A-13B illustrate a perspective view and a side view of a system1300 for analyzing particles. System 1300 includes an inlet 110, athermophoretic particle concentrator 120 and a particle discriminator130. Thermophoretic particle concentrator 120 includes a pair ofthermophoretic heater elements 122 a, 122 b positioned near a peripheryof an air channel 102. The thermophoretic heater elements 122 a, 122 bare configured to thermophoretically force airborne particles in the airchannel 102 away from the periphery and towards an interior region ofthe air channel 102 and air stream 106. The channel walls 104 and thecross-sectional geometry of the air channel 102 may be rectangular.Channel walls 104 include portions of a lower wall 1312, side walls1314, 1316 and upper wall 1318. The perspective view shown in FIG. 13Ahas the upper wall 1318 removed for clarity. An inlet air stream 112enters an opening in the channel walls 104 upstream of the particleconcentrator 120 and exits an opening in the channel walls 104downstream of the particle discriminator 130. The thermophoretic heaterelements 122 a, 122 b may include one or more heater wires 1322 a, 1322b, 1322 c suspended in the air channel 102 with heater posts 1326 a,1326 b. In some implementations, the heater wires 1322 a, 1322 b, 1322 cmay be formed into a wire mesh. Alternatively, heater elements 122 a,122 b may be constructed of thin, partially conductive films on theinterior surfaces of electrically insulated channel walls, ceilings, andfloors. Electrical current may be sent through heater wires 1322 a, 1322b, 1322 c to generate the desired thermal gradient.

One or more of the heater wires 1322 a, 1322 b, 1322 c of heaterelements 122 a, 122 b may be angled with respect to the air channel 102in an inward direction along the air channel 102 and towards an interiorregion of the air stream 106. The thermophoretic heater elements 122 a,122 b are configured to thermophoretically force airborne particles inthe air channel 102 away from the periphery and towards an interiorregion of the air channel 102 and air stream 106. The thermophoreticheater elements 122 a, 122 b allow focusing of particles in the inletair stream 112 into a tighter beam of particles with higher particleconcentration. Some of the dimensions of the air channel 102 in theregion of the particle concentrator 120 may be narrowed to furtherdirect the particles into a narrower beam.

System 1300 includes a particle discriminator 130 with at least oneheater element 132 and at least one particle detector 134 positioned ona lower wall 1312 downstream of the particle concentrator 120 to collectand detect particles in the air stream 106. Heater element 132 mayinclude a heater wire 1332 suspended in the air channel 102 with heaterposts 1336 a, 1336 b. The dimensions of the walls 104 within theparticle discriminator 130 may be narrowed to further concentrate theparticles and to increase the magnitude of the thermal gradient thatresults in an increase of the thermophoretic forces acting on theparticles. In some implementations, dimensions of the channel walls 104in other directions may be widened to slow the speed of airborneparticles and allow more time for the thermophoretic heater elements 132to force and deflect the particles onto the particle detector 134.

In some implementations, the three-dimensional structure of the airchannel 102 and/or the configuration of the various individuallycontrolled focusing heaters and precipitation heaters may includeindividually controlled focusing and precipitating heaters located onthe top, bottom, and sidewalls of the air channel 102 to produce a moreversatile type of airflow particle detecting device. This device may bereconfigured according to local conditions and particle monitoringobjectives. Such reconfiguration may be facilitated by use of suitableprocessors and control software to control the operation of the variousheater elements, particle detectors, and airflow management devices. Theeffective shape of the flow channels constraining the particulate mattermay be programmable to allow compensation for changes in variousenvironmental variables such as air pressure, temperature, and humidity.

FIGS. 14A-14D illustrate top and side views of a system 1400 with arectangular air channel 102 for analyzing particles in an air stream 106and operation thereof. An air stream 106 enters a rectangular airchannel 102 from an inlet air stream 112, traverses the air channel 102through a thermophoretic particle concentrator 120 and a thermophoreticparticle discriminator 130, and exits the air channel 102 through anoutlet air stream 118. Particle concentrator 120 includes a pair ofheater elements 122 a, 122 b positioned on one side of the air channel102 and another pair positioned on an opposite side of the air channel102. Each heater element 122 a, 122 b may have multiple heater segments1422 a, 1422 b, 1422 c with heater segments 1422 a and 1422 c extendingalong the channel wall 104 in a direction substantially parallel to theair stream 106 and heater segments 1422 b extending along the channelwall 104 in a direction that is angled with respect to the air stream106. Particle discriminator 130 fluidically coupled to the air channel102 includes a heater element 132 with heater segments 1432 a extendingparallel to the air stream 106 and a heater segment 1432 b extending ina direction perpendicular to the air stream 106 for concentrating anddeflecting particles 114 in the air stream 106. A plurality of particledetectors 134 a, 134 b, 134 c may be included in the particlediscriminator 130 for collecting, detecting and analyzing particles.Particles 114 traversing the air channel 102 may be concentrated in aninterior region of air stream 106 within the thermophoretic particleconcentrator 120. Particles 114 traversing the thermophoretic particlediscriminator 130 may be deflected and collected on a surface of one ofthe particle detectors 134 a, 134 b, 134 c for detection and analysis.Additional heater elements (not shown) may be positioned abovedownstream particle detectors 134 b, 134 c to allow greater control overthe generated thermal gradients in the air channel 102 and to allow thedownstream heater elements to be operated at a higher temperature thanthe upstream heater elements so that larger particles passed by thefirst particle detector 134 a may be forced onto one or more of thedownstream particle detectors 134 b, 134 c.

FIGS. 15A-15D illustrate top and side views of a system 1500 with anexpanding air channel 102 for analyzing particles 114 in an air stream106 and operation thereof. Particles 114 may enter system 1500 throughan inlet air stream 112 into an inlet 110 and traverse air channel 102through particle concentrator 120 and particle discriminator 130. Theparticles 114 may be concentrated within the particle concentrator 120when heater elements 122 a, 122 b are heated and thermal gradientsgenerated within the air channel 102 thermophoretically force theparticles 114 towards an interior region of the air channel 102. Heaterelements 122 a, 122 b contain heater segments 1522 a, 1522 b in anexpanding region 1560 of the air channel 102 within the particleconcentrator 120. Heater segment 1522 b is oriented in a directionsubstantially perpendicular to the air stream 106 and cooperates withheater segments 1522 a in the expanding region 1560 to concentrate theparticles 114 in an interior region of the air stream 106, even asstreamlines 108 diverge in the expanding region 1560. Particlediscriminator 130 includes a heater element 132 with heater segments1532 a and 1532 b configured to deflect particles 114 onto one or moreparticle detectors 134 a, 134 b, 134 c. Heater segments 1532 a, 1532 bmay be arranged into a “W” configuration to form a thermal potentialwell for retaining divergent particles 114 near the center of the airchannel 102 and to deflect the particles 114 onto one or more particledetectors 134 a, 134 b, 134 c within the particle discriminator 130. Insome implementations, the heater segments 132 a, 132 b of heater element132 may be configured as a sideward “V” to corral particles in the airstream 102 and to deflect the particles onto the particle detectors 134a, 134 b, 134 c. In some implementations, the heater segments 1532 a,1532 b may comprise a plurality of serpentine segments to increase theresistance of the heater element 132 and increase the heater voltageapplied across the heater element 132.

The application of an external force such as centripetal force, can, insome implementations, be used to improve the ability to differentiateand discriminate between different particle sizes. FIG. 16 illustrates atop view of a system 1600 for analyzing particles in an air stream 106including a centrifugal particle separator stage 1660. Particles 114entering system 1600 in an inlet air stream 112 traverse inlet 110,particle concentrator 120, and centrifugal particle separator stage 1660having a curved air channel 102 positioned between the particleconcentrator 120 and a particle discriminator 130. Particles in the airstream 106 may be spatially separated with smaller, lighter particlesstaying near an inside of the air channel 102 and larger, heavierparticles moving towards an outer portion of the centrifugal particleseparator stage 1660. The particle discriminator 130 may include a one-or two-dimensional array of particle detectors 134 configured to detectspatially separated particles from the centrifugal particle separatorstage 1660. The system 1600 may further include an airflow expansionstage 1670 positioned between the centrifugal particle separator stage1660 and the particle discriminator 130. The airflow expansion stage1670 may have an air channel 102 that widens as the air stream 106traverses the airflow expansion stage 1670. Particles spatiallyseparated in the centrifugal particle separator stage 1660 may befurther separated in the airflow expansion stage 1670 as streamlineswithin the airflow expansion stage 1670 diverge. Additionally, the airchannel 102 within the airflow expansion stage 1670 may widen to slowthe air velocity and particle velocity in the air stream 106 as the airstream 106 traverses the airflow expansion stage 1670 to allow more timefor thermophoretic forces to act on and deflect the particles.

Prior to entering the centrifugal particle separator stage 1660,particles may be concentrated in an interior region that is somewhatoffset from a centerline of the air channel 102 in the centrifugalparticle separator stage 1660. The thermophoretic particle concentrator120 may have heater elements 122 a, 122 b with heater segments 1622 a,1622 b, 1622 c configured to force particles in the air stream 106towards an interior region that is offset from the centerline of airchannel 102 within the particle concentrator 120 to utilize more of theair channel 102 in the centrifugal particle separator stage 1660. Toforce particles towards an interior region offset from the centerline,heater segments 1622 a and 1622 c may be extended in a directionparallel to the air channel 102 and air stream 106 with heater segment1622 c positioned closer to a centerline of the air channel 102 in theparticle concentrator 120 and heater segment 1622 b extended in adirection that is angled with respect to the air stream 106.

FIGS. 17A-17B illustrate top and side views of a system 1700 foranalyzing particles with a widening air channel 102 and a narrowingchannel height. System 1700 includes an inlet 110, a thermophoreticparticle concentrator 120 and a thermophoretic particle discriminator130.

The thermophoretic particle concentrator 120 includes an air channel 102between channel walls 104 having a first open end for an inlet airstream 112 and a second open end for an outlet air stream 118. The airchannel 102 is enclosed by channel walls 104 extending from at least thefirst open end to the second open end. Two or more heater elements 122a, 122 b each having at least one heater segment 1722 a may bepositioned between the first open end and the second open end and near aperiphery of the air channel 102. The cross-sectional geometry of theair channel 102 and channel wall 104 perpendicular to the air stream 106is rectangular, and at least two heater elements 122 a, 122 b having atleast one heater segment 1722 a are positioned on two opposing sides ofthe channel wall 104. The heater segments 1722 a of heater elements 122a, 122 b extend along the channel wall 104 in a direction substantiallyparallel to the air stream 106.

The particle discriminator 130 including heater element 132 and particledetectors 134 a, 134 b, 134 c is coupled to the air channel 102 in adirection downstream from the particle concentrator 120. The width ofthe air channel 102 increases and the height of the air channel 102decreases in the downstream direction within the particle discriminator130, allowing the airstream velocity and particle velocity to slow andthe thermal gradient to increase in the vicinity of the particledetectors 134 compared to a cross-sectional geometry of constantdimensions. Extended heater segments 1732 a of heater element 132 extendin a direction nominally parallel to the air stream 106 and areconfigured to retain or further concentrate particles in an interiorregion of the air stream 106, even as the air channel 102 widens. Heatersegment 1732 b of heater element 132 extends in a direction nominallyperpendicular to the air stream 106 near the particle detectors 134 a,134 b, 134 c to allow deflection, collection, detection and analysis ofparticles in the air stream 106. The cross-sectional geometry of the airchannel 102 within the particle discriminator 130 such as the channelheight may be narrowed as the air stream 106 traverses the particlediscriminator 130. The narrower region of the air channel allows lessdistance between the heater segments 1732 b and the particle detectors134 a, 134 b, 134 c, which may result in a higher thermal gradient and alarger thermophoretic force to be generated on particles traversing thenarrowed region.

In operation, smaller particles 1714 a, 1714 b that are concentrated inthe particle concentrator 120 may be deflected in the particlediscriminator 130 by thermal gradients generated by heater segments 1732a, 1732 b of heater element 132 and collected on a surface of a particledetector 134 a. Larger particles 1714 c, 1714 d that are concentrated inthe particle concentrator 120 may then be deflected by thermal gradientsgenerated by heater segments 1732 a, 1732 b of heater element 132 andcollected on a surface of a particle detector 134 b or particle detector134 c. Controlling the velocity of the air stream 106 and the thermalgradients generated in the particle discriminator 130 allow forselective deflection and collection of particles in a particularparticle size range onto a surface of one of the particle detectors 134a, 134 b, 134 c.

FIGS. 18A-18B illustrate top and side views of a thermophoretic particledetection system 1800 for analyzing particles 114 including a pair ofmulti-tapped heater elements 122 a, 122 b extending through the particleconcentrator 120 and the particle discriminator 130. An air channel 102enclosed by channel walls 104 extends through the inlet 110, particleconcentrator 120 and particle discriminator 130. The multi-tapped heaterelements 122 a, 122 b have heater segments 1822 a, 1822 b, 1822 cextending through the particle concentrator 120 and additional heatersegments 1822 d extending through the particle discriminator 130. Stubheater segments 1822 e may be provided to allow additional control overthe temperature profile and thermal gradients generated in the airchannel 102 and air stream 106. Heater segments 1822 d extend nominallyparallel to the air stream 106 in the particle discriminator 130 whereasheater segments 1832 a, 1832 b, 1832 c extend in a direction nominallyperpendicular to the air stream 106 on a side opposite the particledetectors 134 a, 134 b, 134 c to allow deflection, collection,detection, and analysis of particles entering through the inlet airstream 112.

Heater taps 1828 a through 1828 n provide electrical connections to thevarious heater segments in heater elements 122 a, 122 b for selectiveapplication of electrical power to allow control over the heat generatedby each heater segment and the thermal gradients generated in the airchannel 102. Controlling the velocity of the air stream 106 and thethermal gradients generated in the particle concentrator 120 and theparticle discriminator 130 allows for selective concentration,deflection, and collection of particles 114 onto a surface of one of theparticle detectors 134 a, 134 b, 134 c. The multi-tapped heater elements122 a, 122 b allow continuous heater segments, local temperature zonecontrol, a configurable temperature profile, reduced electrical leadoutrequirements, and a flexible target particle response. In someimplementations, multi-tapped heater elements 122 a, 122 b may comprisea thermally isolated wall-mounted thin-film heater element with apolymeric barrier layer that serves as a channel wall for the airchannel 102. In some implementations, one or more heater segments ofheater elements 122 a, 122 b may be modulated with a varying voltage toallow controlled scanning of particles 114 in a longitudinal directionor a lateral direction with respect to the air stream 106 so thatparticles of a selected particle size range may be collected on one ofthe particle detectors 134 a, 134 b, 134 c. Modulation of voltagesapplied across stub heater segments 1822 e and to deflection heatersegments 1832 a, 1832 b, 1832 c may aid in particle selection,deflection, fractionation, detection, and collection uniformityimprovements.

The service lifetime of thermophoretic particle detection system 1800may be extended by controlling and limiting the amount of particulatematter collected on the surface of the particle detectors 134 a, 134 b,134 c. For example, particle detector 134 a may be operated inconjunction with overlying deflection heater segment 1832 a to collectparticulate matter while particle detectors 134 b, 134 c and associatedheater segments 1832 b, 1832 c remain in an off condition. After aselected time period of seconds, minutes, hours or days depending on theapplication, particle detector 134 a and deflection heater segment 1832a may be turned off and particle detector 134 b with deflection heatersegment 1832 b may be turned on and put into operation. After anotherselected time period, particle detector 134 b and deflection heatersegment 1832 b may be turned off and particle detector 134 c withdeflection heater segment 1832 c may be put into operation, and soforth. In this manner, the power applied to each of the heater segments1832 a, 1832 b, 1832 c may be controlled to allow scanning of thedeflected particles toward a peripheral wall and onto a surface of theparticle detectors 134 a, 134 b, 134 c. Power (e.g. electrical power)applied to one or more of the heater segments 1822 c, 1822 d or 1822 emay be controlled to deflect particles in a lateral direction along theperipheral wall. In some implementations, the incremental change in theresonant frequency or the shift in frequency from a baseline value forresonant-based particle detectors may be used to determine which pair ofparticle detectors and heater segments are operated and for how long.

FIG. 19 illustrates a block diagram of a system 1900 for analyzingparticles in an air stream. Particle detection system 1900 includes acontroller 1910 with one or more processors and circuitry for runningprogram code and executing instructions to analyze particles in an airstream among other functions. Controller 1910 may be connected via acommunications bus 1940 to one or more memories 1912. Memory 1912 mayinclude a combination of volatile and non-volatile memory for storingprogram instructions and data. Controller 1910 may communicate withother processors and data systems external to system 1900 via one ormore wireless communication links 1914 and antennas 1916 or one or morewired communication links 1918 and external communication lines 1920such as Ethernet or USB connections. One or more power sources 1922 andground lines 1924 such as batteries or AC/DC power connections mayprovide local regulated power for devices connected to bus 1940. Varioussensors 1926 and transducers such as temperature sensors, pressuresensors, humidity sensors, accelerometers, gyroscopes, ambient lightsensors, clocks, microphones, and speakers may be connected tocontroller 1910 via communications bus 1940.

One or more particle detection modules 1930 for detecting particles inan air stream may include an inlet air stream 112 for incoming sampleair and an outlet air stream 118 for outgoing air. The particledetection module 1930 may include one or more inlets, thermophoreticparticle concentrators, and thermophoretic particle discriminators. Theair stream within the particle detection module 1930 may be encompassedby the walls of an air channel extending from a first open end for theinlet air stream to a second open end for the outlet air stream. Theparticle detection module 1930 may be connected to controller 1910 viacommunications bus 1940 or other dedicated control and/or data lines.Controller 1910 may send control signals to control the power applied tovarious heater elements coupled to the air stream in the particledetection module 1930. Controller 1910 may be coupled to one or more airmovement devices for controlling the movement of air through the airchannel.

In some implementations, controller 1910 may provide one or more controlsignals to particle detection module 1930 to generate and adjust thermalgradients in the air stream. For example, thermal gradients in the airstream may be adjusted by adjusting power applied to one or more heaterelements that generate the thermal gradient or by adjusting an airstreamvelocity of the air stream in the air channel.

FIG. 20 shows a block diagram of a method 2000 for analyzing particlesin an air stream. The method 2000 includes applying power to heaterelements positioned on various sides of an air channel encompassing atleast a portion of the air stream, as shown in block 2005. Power may beapplied to one or more pairs of heater elements that may be positionednear a periphery and on opposite sides of the air channel. In someimplementations, the entire length of the air channel in thethermophoretic particle concentrator functions as a heater. In otherimplementations only short portions of the air channel function as aheater. In other implementations, sets or arrays of heater elements maybe employed at certain sections of the air channel. These heaterelements may operate at different temperatures and may be individuallyaddressed in order to provide a high degree of flexibility in thegenerated thermal gradient.

In some implementations, the power to the heater elements may be dutycycled (turned on and off) to extend the lifetime of system components.In many use cases, the time constant associated with any significantchange in particulate matter concentration is on the order of tens ofseconds to minutes or hours or more. Since air quality measurements mayonly be needed to be conducted once every few seconds or few minutes, orevery few hours, there may be extended periods of time during whichsampling of particulate matter may be turned off.

Air may be drawn through the air channel, as shown in block 2010. Thedrawn air may generate the air stream within the air channel. Air may bedrawn through the air channel using any one of a variety of air movementdevices such as a pump, blower, fan, turbine, motorized air intakedevice, bellows pump, membrane pump, peristaltic pump, piston pump,positive-displacement pump, rotary vane pump, Venturi device, airflowmanagement device, or other air drawing means for moving or drawing airthrough the air channel. Drawing air through the air channel may beperformed with a duty cycle corresponding approximately with the dutycycling of the heater elements.

Thermal gradients may be generated within the air channel, as shown inblock 2015. Heat from electrical power applied to the heater elementscombined with airflow profiles and air channel geometries generate oneor more thermal gradients within the air channel, resulting inthermophoretic forces on particles in the air stream directed mainlytowards the interior or center of the air stream.

Particles in the air stream may be forced away from the periphery of theair channel and towards an interior region of the air channel with thethermophoretic force generated by the thermal gradient to concentratethe particles in an interior region of the air stream, as shown in block2020. Aerosol particles introduced into the inlet of the air channel maybe distributed somewhat randomly throughout the cross-sectional area ofthe air stream. Action by the thermophoretic particle concentrator mayreduce the physical cross-section and narrow the distribution of theparticles flowing in the air stream as the air stream and the particlestraverse the particle concentrator through the use of controlled thermalgradients. Particle concentration may be achieved through the use ofopposing thermophoretic forces aligned with respect to one or more axesof the air channel.

The generated thermal gradients are dependent in part on the loss ofheat into the air stream. The air stream in the air channel may exhibita velocity gradient as a function of distance from the channel wall andlength down the channel. Since the amount of heat removed is a functionof the local velocity of air in the air stream, the generated thermalgradients are functionally dependent on the airstream velocity profile.

Particles concentrated in the air stream may be detected, as shown inblock 2025. In some implementations, particles may be detected bydeflecting the particles with generated thermophoretic forces to directparticles in the air stream away from the interior region of the airchannel and towards one or more particle detectors positioned on a wallof the air channel, where the particles may be collected on a surface ofthe particle detector and cause a change in a resonant frequency of theparticle detector in response to the mass loading on the surface. Insome implementations, the change in resonant frequency over a fixed timemay be determined as an indication of the effective mass added onto thesurface of the particle detector. In some implementations of particularbenefit in environments with a large particulate matter concentration,an adaptive cycle may be used that measures the time to depositparticulate matter on a resonant-based particle detector for apredetermined frequency shift. The system may use at least one processorand be under software control so that when the air particle density ishigh, the unit may sample less frequently in order to extend thelifetime of the sensor.

In some implementations, the thermal gradients in either the particleconcentrator or the particle discriminator may be modulated bymodulating the power to the associated heater elements. Modulation ofthe thermal gradients may spread out the deposition of particles on theparticle detectors to avoid non-uniform deposition and to extend thelifetime of the particle detectors.

The detected particles in the air stream may be analyzed, as shown inblock 2030. One or more algorithms may be applied to detect thefrequency shift of the resonant particle detector and to compensate fortemperature effects. The algorithm may apply calibration coefficientsand various model parameters to determine an effective mass of theparticles collected on the surface of the particle detector and togenerate an aerosol mass concentration estimate for the sampled air. Insome implementations, the aerosol mass concentration may be estimatedfor one or more selected particle size ranges.

FIG. 21 shows a block diagram of a method 2100 for analyzing particlesincluding the generation of an aerosol mass concentration estimate.Method 2100 includes concentrating particles in an interior region of anair stream, as shown in block 2105. Particles in the air stream may beconcentrated with a thermophoretic concentrator having a plurality ofheater elements. The particles may traverse the thermophoreticconcentrator and enter a thermophoretic particle discriminator. Thethermophoretic particle discriminator may have one or more heaterelements. The thermal gradient within the particle discriminator may becontrolled, as shown in block 2110. The thermal gradient may begenerated and controlled by controlling the amount of electrical powerapplied to each of the heater segments and heater elements, whileaccounting for the airstream velocity in the air stream.

An airstream velocity of the air stream within the particlediscriminator may be controlled, as shown in block 2115. The airstreamvelocity may be controlled and adjusted as needed by sending controlsignals and controlling the power to an air movement device normallypositioned downstream and fluidically coupled to the air channelencompassing the air stream. For example, increasing the flow rate ofgas through the air movement device increases the airstream velocity andvelocity distribution of air or other gas drawn through the air channel.

Method 2100 may include deflecting the concentrated particles in the airstream with a generated thermal gradient, as shown in block 2120.Smaller particles in the air stream may be selectively deflected awayfrom the interior region and towards a periphery of the air stream at adifferent rate than larger particles in the air stream. The thermalgradient generated in the particle discriminator may be controlled todeflect particles in a selected particle size range onto a surface ofone or more particle detectors. Deflected particles within a selectedparticle size range may be collected on a surface of a particledetector, as shown in block 2125. The selected particle size range mayinclude one of a particle size range between about 0.01 microns and 0.1microns, 0.01 microns and 0.3 microns, 0.1 microns and 1.0 microns, 1.0microns and 2.5 microns, 2.5 microns and 10.0 microns, and 10.0 micronsand larger. The particle detector may be one or more of a bulk acousticwave (BAW) resonator, a thin-film bulk acoustic wave resonator (FBAR), asolidly mounted resonator (SMR), a quartz crystal microbalance (QCM), awall-mounted particle detector, a time-of-flight detector, a resonantsensor, a capacitive sensor, an infrared sensor, an optical sensor, a UVsensor, or a particle mass detector.

Implementations with more than one precipitation heater element mayoperate in a configuration with a plurality of spatially separatedprecipitation heating zones, each configurable to operate at the same ordifferent temperatures. The precipitation heat gradients in eachprecipitation heating zone may be configured to increase as a functionof distance along the air channel as the airborne particles continue tomove downstream. The smallest particles may be the first to be propelledout of the air stream towards a particle detector when the particlesfirst encounter a low-intensity precipitation heating zone, while mediumand larger particles may continue on in the air stream. The largerparticles, which may not have been significantly defected during thefirst, low-intensity precipitation heating zone, may encounter a moreintense precipitation heating zone with a higher thermal gradient.Larger particles may be increasingly deflected and propelled towardsanother particle detector as the larger particles encounter a later,higher intensity part of the precipitation heat gradient. The net effectmay further increase the ability to discriminate between airborneparticles of different sizes and other characteristics.

An effective mass of the particles collected on the surface of one ormore particle detectors may be determined, as shown in block 2130. Insome implementations, the effective mass of particles collected on thesurface may be the actual mass of the collected particles or anon-normalized estimate of the actual mass. In some implementations, theeffective mass may be computed as the difference in mass between acurrent air sample and an earlier air sample. In some implementations,the effective mass may be normalized to the effective surface area ofthe particle detector. For example, an estimate of the effective massmay be determined from a form of the Sauerbrey equation that relateschanges in frequency of a resonant sensor to the amount of added mass bytaking into account the density and elastic moduli of the resonatorbody, the active surface area of the resonator, and the baselineresonant frequency of the resonator.

An aerosol mass concentration estimate of the particles within theselected particle size range may be generated, as shown in block 2135.For example, the effective mass of particles collected from an airsample may be combined with knowledge of the airflow rate and channelgeometry along with the time allocated for sampling to generate theaerosol mass concentration estimate. In some implementations, theaerosol mass concentration estimate may be computed with appropriatelyscaled multipliers to provide the estimate in the preferred units ofmicrograms per cubic meter or other selected set of units. In someimplementations, the aerosol mass concentration estimate may begenerated for particles in the selected particle size range or in a setof selected particle size ranges.

The aerosol mass concentration estimate may be sent or otherwiseprovided to a requesting entity, as shown in block 2140. In someimplementations, the requesting entity may be a user of the particledetection system, an electronic device in communication with theparticle detection system, or a database in a cloud-based data centerwhere temporal and spatial aggregations of particulate matterconcentrations from a multiplicity of thermophoretic particle detectionsystems in a geographical region may be maintained over an extendedtime.

Although the various blocks and steps described in the above processflows and methods are intended to be representative, the steps and theorder of the steps may be altered and still remain within the scope,spirit, and claims of this disclosure. Variations in the steps and theorder of the steps may be made without loss of generality, such asperforming one step before another or combining two or more steps intoone step.

While various implementations have been described above, it should beunderstood that the implementations have been presented by way ofexample and not limitation. The breadth and scope of the presentdisclosure should not be limited by any of the implementations describedabove but should be defined in accordance with the following claims,subsequently submitted claims and their equivalents.

1. A method of analyzing particles in an air stream, the methodcomprising: concentrating particles in an interior region of the airstream; and deflecting said concentrated particles in the air streamwith a generated thermal gradient; wherein smaller particles in the airstream are selectively deflected away from the interior region andtowards a periphery of the air stream at a different rate than largerparticles in the air stream.
 2. The method of claim 1, furthercomprising: controlling the generated thermal gradient to deflectparticles in a selected particle size range onto a surface of a particledetector.
 3. The method of claim 1, further comprising: controlling anairstream velocity of the air stream.
 4. The method of claim 1, furthercomprising: collecting deflected particles within a selected particlesize range on a surface of a particle detector.
 5. The method of claim4, wherein the selected particle size range includes one of a particlesize range between about 0.01 microns and 0.1 microns, 0.01 microns and0.3 microns, 0.1 microns and 1.0 microns, 1.0 microns and 2.5 microns,2.5 microns and 10.0 microns, and 10.0 microns and larger.
 6. The methodof claim 4, wherein the particle detector is one of a bulk acoustic wave(BAW) resonator, a thin-film bulk acoustic wave resonator (FBAR), asolidly mounted resonator (SMR), a quartz crystal microbalance (QCM),wall-mounted particle detector, a time-of-flight detector, a resonantsensor, a capacitive sensor, an infrared sensor, an optical sensor, a UVsensor, or a particle mass detector.
 7. The method of claim 4, furthercomprising: determining an effective mass of the particles collected onthe surface of the particle detector.
 8. The method of claim 7, furthercomprising: generating an aerosol mass concentration estimate of theparticles within the selected particle size range; and providing theaerosol mass concentration estimate.
 9. A system for analyzingparticles, the system comprising: an inlet; a particle concentratorfluidically coupled to the inlet; a particle discriminator fluidicallycoupled to the particle concentrator, the particle discriminatorincluding an air channel for containing an air stream, the air channelextending from the inlet through the particle concentrator and throughthe particle discriminator; and a controller electrically coupled to theparticle concentrator and to the particle discriminator, the controllerconfigured to allow: concentrating particles in an interior region ofthe air stream; and deflecting the concentrated particles in the airstream with a generated thermal gradient; wherein smaller particles inthe air stream are selectively deflected away from the interior regionand towards a periphery of the air stream at a different rate thanlarger particles in the air stream.
 10. The system of claim 9, thecontroller further configured to allow: controlling the generatedthermal gradient to deflect particles in a selected particle size rangeonto a surface of a particle detector.
 11. The system of claim 9, thecontroller further configured to allow: controlling an airstreamvelocity of the air stream in the air channel.
 12. The system of claim9, the controller further configured to allow: collecting deflectedparticles within a selected particle size range on a surface of aparticle detector.
 13. The system of claim 12, the controller furtherconfigured to allow: determining an effective mass of the particlescollected on the surface of the particle detector.
 14. The system ofclaim 13, the controller further configured to allow: generating anaerosol mass concentration estimate of the particles within the selectedparticle size range; and providing the aerosol mass concentrationestimate.
 15. A non-transitory computer-readable medium storingcomputer-readable program code to be executed by at least one processorfor analyzing particles in an air stream, said program code comprisinginstructions configured to cause: concentrating particles in an interiorregion of the air stream; and deflecting said concentrated particles inthe air stream with a generated thermal gradient; wherein smallerparticles in the air stream are selectively deflected away from theinterior region and towards a periphery of the air stream at a differentrate than larger particles in the air stream.
 16. The non-transitorycomputer-readable medium of claim 15, the instructions furtherconfigured to cause: controlling the generated thermal gradient todeflect particles in a selected particle size range onto a surface of aparticle detector.
 17. The non-transitory computer-readable medium ofclaim 15, the instructions further configured to cause: controlling anairstream velocity of the air stream in an air channel.
 18. Thenon-transitory computer-readable medium of claim 15, the instructionsfurther configured to cause: collecting particles within a selectedparticle size range on a surface of a particle detector.
 19. Thenon-transitory computer-readable medium of claim 18, the instructionsfurther configured to cause: determining an effective mass of theparticles collected on the surface of the particle detector.
 20. Thenon-transitory computer-readable medium of claim 19, the instructionsfurther configured to cause: generating an aerosol mass concentrationestimate of the particles within the selected particle size range; andproviding the aerosol mass concentration estimate.